Shallow structure and its formation process of an active flexure in the forearc basin of the central Nankai subduction zone

ENE-WSW trending active faults, named Enshu fault system, are developed in the forearc basins of the eastern and central Nankai subduction zone. Three parallel faults developed in the Enshu forearc basin of the eastern Nankai have right lateral slip on the basis of dextral displacement of the canyon axis. Moreover, bathymetry data and side-scan sonar imageries indicate relative uplift of the northern region and the multichannel seismic (MCS) reflection profiles show northward dipping fault planes. In the central Nankai subuduction zone, an ENE-WSW trending step is distributed at the northern part of the Kumano forearc basin and is regarded as the western extension of the Enshu fault system. Although MCS records show deformations including an anticlinal fold beneath the bathymetric step, they have less resolution to identify deformation of basin sequence just below the seafloor. In contrast, deformation seems to reach to the seafloor on a profile by SBP mounted on a mother ship. Investigation of shallow deformation structures is significant for understanding of recent tectonic activity. We carried out deep towed SBP survey by ROV NSS (Navigable Sampling System) during Hakuho-maru KH-11-9 cruise. High resolution mapping of shallow structures was successfully conducted by a chirp SBP system of EdgeTech DW-106. ROV NSS also has capability to take a long core with a pinpoint accuracy around complex topographic region. The Kumano forearc basin is topographically divided into the northern part at a water depth of 2038 m and the other major region at a depth of 2042 m by the ENE-WSW linear step. Three deep towed SBP lines intersected this topographical step and revealed the following structures. This step is composed of 100 m wide gentle slope with an inclination of about 8 degrees. An anticlinal axis is located beneath the upper edge of this slope. Sedimentary layers continue at this slope region without any abut/termination and rapidly increase their thickness toward the seaward and the landward of the slope. This suggests that the anticlinal ridge trapped sediments from the landward region, and overflowed sediments thinly covered the slope and filled the basin floor seaward of it. Because the upper 25 m sequence recognized by deep towed SBP shows no fault deformation, the step is interpreted to be caused by flexure deformation. An acoustically transparent layer is observed in this area. The thickness of this layer is 1 m at the slope and 5 m at the other regions. Four core samples indicate that the transparent layer correspond to the sequence younger than 10,000 years ago. The sequence landward of the slope indicates growth strata: thinning toward the anticlinal axis and increase of tilt angle downward. This structure is recognized from the seafloor to the strata below the transparent layer suggesting continuous deformation to the present. It is inferred that the flexure structure observed on the deep towed SBP data was formed by a landward dipping thrust fault estimated on the MCS profiles. Flower structures on the MCS data also suggest strike slip displacement and are consistent to the deformation in the Enshu forearc basin 80 km northeast of the study area.